wieso nicht

...

ungefähr so

  
  
 var zeit=0;  
 function countstop(){  
  zeit=(new Date).getTime()-zeit;  
  document.getElementById("downloading").innerHTML=(180000/zeit)+" pro Minute";  
 }  
 function countstart(){  
  zeit=(new Date).getTime();  
 }  

  
  
<input type="button" name="button" value="Start" onclick="countstart()" />  
<input type="button" name="button" value="Stop" onclick="countstop()" />  
<br>  
<span id="downloading">nix</span>